They’re asking if they use the same transfer. If they do, then the US bluray is the one worth working from, as it’s 1080p progressive and can be sped up to 25fps easily, whereas the AUS BD would have to be deinterlaced. (the BD spec only allows 25/50fps in an interlaced form.)
True, but if the source is truly progressive, then the 1080i50 stream will be identical to what a 1080p25 would’ve been if it were a permitted format. If there is any hint of interlacing (such as scrolling credits) then it would’ve been intended on the original version. There would be no interlacing created by making it 1080i50,unlike NTSC would have.